Lightbox overlay / modal adds padding to right of page

I added a lightbox gallery to a page. When I click one of the lightbox thumbnails to launch the overlay (modal), extra padding is added to the entire right side of the page behind the overlay.

Here’s a video: https://www.useloom.com/share/558512f56b514f3aa3e62547d814a0c8

Anyone else seeing this?